Demodulator and method for demodulating a DQPSK-modulated optical signal

摘要

A demodulator (1a) for demodulating a DQPSK-modulated optical signal (a(t)), comprises: a differential interferometer (4) having a first path (4.3) and a second path (4.4), delay means (4.5) for producing a temporal delay (τ) of the optical signal between the paths, and phase shifting means (4.6) for producing a phase shift (Ψ) of the optical signal between the paths, wherein the temporal delay substantially corresponds to a symbol period of the optical signal; a balanced detector (5) connected to outputs (4.7, 4.8) of the differential interferometer to produce an electrical output signal (i(t)); and a sampling unit (7.1-7.3) connected to an output of the balanced detector and adapted to sample the electrical output signal at samplings instants, kTs+ΔT, k = 0, 1,..., a temporal spacing of which substantially corresponds to a symbol period of the optical signal a(t)), thus producing a sampled electrical output signal, ik. In the proposed demodulator, the phase shift (Ψ) between the paths (4.3, 4.4) of the differential interferometer (4) is set so that the sampled electrical output signal (ik) has four different detectable amplitude levels (L1-L4).